This invention relates to an element and module of hollow fibre filtering membranes of the type normally immersed in a tank and used to withdraw a filtered permeate by means of suction applied to the lumens of the membranes.
The inventors herein and others developed a module of filtering hollow fibre membranes which they described in U.S. Pat. No. 5,248,424, issued on Sep. 28, 1993. In this module, hollow fibre membranes are held in fluid communication with a pair of horizontally spaced headers to form modules in a variety of configurations in which the fibres vary from being substantially horizontal to substantially vertical. To produce permeate, transmembrane pressure (xe2x80x9cTMPxe2x80x9d) is provided by suction on the lumens of the fibres.
Subsequently, further shell-less membrane modules based in part on similar principles appeared with hollow fibre membranes in both substantially vertical and substantially horizontal orientations. Shell-less modules with membranes oriented vertically are shown in U.S. Pat. No. 5,639,373 issued to Zenon Environmental Inc. on Jun. 17, 1997; U.S. Pat. No. 5,783,083 issued to Zenon Environmental Inc. on Jul. 21, 1998 and PCT Publication No. WO 98/28066 filed on Dec. 18, 1997 by Memtec America Corporation. In these modules, the horizontally spaced headers are replaced by headers spaced vertically only.
Shell-less modules with membranes oriented horizontally are described, for example, in U.S. Pat. No. 5,480,553 issued to Mitsubishi Rayon Co., Ltd on Jan. 2, 1996; European published application EP 0,931,582 filed on Aug. 8, 1997 by Mitsubishi Rayon Co., Ltd and in an article, xe2x80x9cDevelopment of a tank-submerged type membrane filtration systemxe2x80x9d, by K. Suda et. al. of Ebara Corporation published in Desalination 119 (1998) 151-158.
Despite these developments, membrane filtration technology is not widely used for creating potable water. Sand filters are still used more often, largely because of their lower cost for a given capacity. For example, tests by the Ebara Corporation were reported in the article mentioned above. While the authors achieved stable operation over extended periods of time, the tank superficial velocity (the flux of permeate, typically in m3/h, divided by the tank footprint, typically in m2) was only about 1.7 m/h. In comparison, a typical sand filtration system has a tank superficial velocity of 5-10 m/h allowing for the use of much smaller tanks, a significant cost in a large municipal or industrial system. Modules of vertical membranes produced by Zenon Environmental Inc. have been operated to produce tank superficial velocities of over 10 m/h but while using strong membranes supported with a substrate and fairly intense aeration. Both the complex membranes and the intense aeration increase the cost of such technology. Finally, the cost of maintaining known membrane modules is also a concern. In particular, leaks or defects in a single membrane are difficult to locate or isolate and often require large sections of a filtration system to be taken off line for their repair.
It is an object of the present invention to provide a hollow fibre filtering membrane element and module. It is another object of the present invention to provide a process which uses immersed filtering membranes, particularly as part of a process of producing potable water.
in one aspect, the invention is directed at an element having hollow fibre membranes attached to and suspended between a pair of opposed horizontally spaced, vertically extending headers. Side plates extending between the pair of vertically extending headers define a vertical flow channel through the element. The hollow fibre membranes are arranged in bundles which, when dispersed, fill a central portion of the vertical flow channel. Preferably, only one header of the pair of headers has a permeate channel and the hollow fibre membranes are fixedly attached to the other header.
A module of filtering hollow fibre membranes is made by arranging such elements side by side or in an orthogonal grid such that the side plates and headers of the elements form a plurality of directly adjacent vertical flow channels. A frame restrains the elements in place without obstructing the vertical flow channels. The restraint provided by the frame may be released for a selected element, however, allowing the selected element to be removed or replaced in a direction substantially normal to its headers without disassembling the remainder of the module. Each element has an associated releasable and resealable water tight fitting between the element and a permeate collector, the releasable water tight fitting being released when the element is removed from the module. An aerator below the module has a plurality of air holes located to provide a line of air holes below each element or below a side plate between each pair of elements.
With such modules, membrane defects can be accurately located by a bubble point test applied to each row of the module in turn. The presence of bubbles in a particular flow channel indicates which, if any, elements of a row are defective. Once located, a defective element is replaced with a new element allowing permeation to resume while the defective element is repaired.
In another aspect, the invention is directed at a process for filtering water using such elements or modules. In the process, permeate flux is less than 50 L/m2/h, preferably less than 35 L/m2/h, and aeration to scrub the membranes is provided when permeation is periodically stopped. A tank containing the elements or modules is emptied and refilled from time to time to remove accumulated solids. Gentler aeration may be provided during permeation to homogenize the contents of the tank. Such a process is both gentle on the membranes and energy efficient. Use of the process and element design of the invention allows inexpensive membranes to be used to create elements having a membrane surface area of at least 500 m2 for every cubic metre of element volume and provides good yield at acceptable energy costs.